In recent mobile communications, there has been an increase in the distribution of “rich” content such as Internet access, streaming broadcasts, music and video, and other content in place of the voice-centered communication of conventional portable telephones. This has been accompanied by demands for faster access and improved throughput performance.
In order to address such demands, much energy is being devoted to the introduction and study of multi-antenna systems using a plurality of transmission/reception antennas in various mobile communication systems. Multi-antenna systems employ various methods, such as diversity methods, beamforming methods, and MIMO (Multi-Input, Multi-Output) methods; and methods for use are selected taking the environment, the application, and other factors into consideration.
Further, when employing a multi-antenna system in communication from a wireless base station to a mobile terminal station in mobile communications, it has for example been proposed that the velocity of motion of the mobile terminal station be estimated and the multi-antenna communication method selected, and that by this means the optimum diversity gain is obtained (see Patent Reference 1).
One multi-antenna communication method which is a method of controlling a multi-antenna system is the closed-loop method, in which information on the state of reception of forward-link transmission signals transmitted from a wireless base station are fed back from a mobile terminal station to the wireless base station, and by performing transmission at the wireless base station which reflects this reception state information, high performance is obtained.
On the other hand, an open-loop method not requiring feedback information is another method of control of multi-antenna systems. Transmission diversity employing Space-Time Coding and other techniques are employed in open-loop methods.
Other categories of methods include tracking control methods, in which each antenna is controlled so as to track relatively rapid changes in the propagation channel, and direction control methods, in which high correlation between individual antennas is assumed, and control is executed such that the propagation path direction of mobile terminal stations is tracked by the directionality formed by a plurality of antennas.
In general, closed-loop methods enable satisfactory characteristics when optimal feedback information can be utilized, and are desirable for requiring a small amount of processing on the side of the mobile terminal station. However, such methods have the drawback that there is prominent degradation of characteristics when the timing of use of the feedback information is affected by changes in the propagation channel state relative to the timing of measurement of the state of the forward-link transmission signal by the mobile terminal station.
To facilitate understanding, figures are used to explain the above in further detail.
FIG. 1 is a conceptual diagram of a mobile communication system to which the invention may be applied. The wireless base station 10 performs communication with the mobile terminal stations 11a and 11b (hereafter, when there is no need for discrimination, simply called mobile terminal stations 11), either directly without passing through a wireless relay station, or via a wireless relay station 12.
Transmissions from the wireless base station 10 to a mobile terminal station 11 through a downlink channel are called forward-link communications FC, and transmissions from a mobile terminal station 11 to the wireless base station 10 through an uplink channel are called reverse-link communications RC.
When performing communication using multi-antennas, the optimal multi-antenna communication method will differ depending on the method of antenna installation, the signal propagation environment, and other factors. And when using closed-loop multi-antenna control, the optimal method will differ depending on the delay time until the feedback information is applied.
In general, compared with the wireless base station 10, a mobile terminal station 11 has low antenna gain due to limitations on the terminal size, and transmission power is also limited. For these reasons, there are cases in which the coverage of reverse-link communications RC is smaller than the coverage 13 of forward-link communications FC.    Patent Reference 1: Japanese Laid-open Patent Publication No. 2004-40801